Project Summary/Abstract: The objective of the proposed research is to develop synthetic models of proposed reactive sites within the oxygen-evolving complex (OEC) of photosystem II to investigate properties of molecular and electronic structure that promote O?O bond formation within photosynthetic water oxidation. The mechanism for this key reaction step has yet to be fully defined, and high-valent manganese-oxido (Mn-O) units have been broadly proposed to participate directly in O?O bond formation within the OEC, most likely proceeding through transient Mn-peroxido intermediates. By taking advantage of a newly engineered tripodal phosphinamide ligand, this research seeks to access discrete, high-valent Mn-oxido/hydroxido (Mn-O or Mn-OH) species and examine their reactivity with external substrates to form Mn-peroxido units and thus evaluate their propensity to engage in O?O bond formation. Our new phosphinamide ligand framework possesses characteristics that aid in stabilization of high-valent metal centers as well as enable binding of auxiliary Lewis acidic metal ions, making it an excellent candidate for stabilizing high-valent, terminal mononuclear Mn-O or Mn-OH species as well as heterobimetallic complexes in which Mn centers and CaII ions are bridged by oxido or hydroxido ligands. This will be of key importance in evaluating competing proposals for O?O bond formation pathways within the OEC as the role of CaII is not yet concretely defined. Rigorous characterization of molecular and electronic structure of mononuclear and heterobimetallic complexes supported by this phosphinamide ligand will be directly compared to analogous compounds supported by urea- and sulfonamide-containing ligands to enable evaluation of ligand effects on electronic structure and redox properties of the resulting complexes. Furthermore, low temperature spectroscopic techniques will be employed to detect and characterize Mn-peroxido species resulting from both mononuclear and heterobimetallic precursors.